1. Field of Invention
The present invention relates to light emitting device. More particularly, the present invention relates to a structure of light emitting diode having a superlattices contact layer. The light emitting diode is a nitride-base III-N group compound semiconductor device.
2. Description of Related Art
In recent years, gallium nitride-based III-N group compound semiconductor device, such as GaN, GaAlN, and GaInN, has been greatly taken as a light emitting device. FIG. 1 is a cross-sectional view, schematically illustrating structure of a conventional light emitting diode made of III-N group compound.
In FIG. 1, the light emitting diode is formed on a substrate 10, such as an Al2O3 substrate. A nucleation layer 12 and an N-type conductive buffer layer 14 are sequentially formed over the substrate 10. The buffer layer 14 includes GaN doped with N-type dopant, so as to ease the crystal growth for the subsequent crystal growing process. There is a light-emitting active layer 18 over the buffer layer 14. Usually, the active layer 18 is confined by a confinement layer, that is, cladding layers 16, 20. The cladding layers 16, 20 are doped with opposite conductive type. For example, if the lower cladding layer 16 is the GaN layer doped with N-type dopants, the upper cladding layer 20 is the GaN layer doped with P-type dopants. Then, a contact layer 22 is formed on the upper cladding layer 20. The contact layer 22 a P-type GaN layer. A transparent electrode layer 24 is formed on the contact layer 22, where the transparent electrode layer usually includes a N-type material layer, such as indium tin oxide (ITO), cadmium tin oxide (CTO), or ultra-thin metal. The transparent electrode serves as an anode of the diode. Moreover, an electrode layer 26, serving as a cathode of the diode, is also formed on the buffer layer 14 but is separated from the cladding layers 16, 20 and the active layer 18.
FIG. 2 is a cross-sectional views, schematically illustrating a light emitting region for the light emitting diode in FIG. 1. In FIG. 2, when the electrodes 24, 26 are applied with a forward bias, the diode is conducted. At this situation, current can flow from the electrode 24 to the active layer 18. In the conventional manner, the P-type contact layer 22 of GaN cannot have high carrier concentration and has large contact resistance. This results in a poor quality of current spreading. The p-type electrode layer 24 also only covers a portion of the contact layer 22. As shown in FIG. 2, the area having current flow is about the width L of the electrode layer 24. This limits the light emitting area for the diode. The function of the active layer cannot be fully performed. The light emitting efficiency of the diode is then greatly reduced.
The conventional LED using AlGaInP compound semiconductor can also be a structure as shown in FIG. 6. In FIG. 6, a substrate layer 602 is used. An n-electrode layer 600 is formed on the back side of the substrate layer 602 while a confinement layer 604 is formed on the upper side of the substrate layer 602. An active layer 606 is formed on the confinement layer 604. Another confinement layer 608 is formed on the active layer 606. Another substrate layer 610 is formed on the confinement layer 608. Then, a p-electrode 612 is form on the substrate layer 610. In this convention structure of LED, the p-electrode 612 directly contacts with the substrate 610 by only a portion. In this structure, the electrode contact is poor. As a result, the light emitting efficiency is poor either. It should be noted that the substrate layer 610 is a semiconductor layer in general.
In summary, the conventional light emitting diode is restricted by the physical properties of the contact layer. The contact layer cannot be grown with a high hole concentration. This also causes the high fabrication cost and also causes low yield. Further still, the conventional structure cannot provide a diode with high light emitting efficiency. A large portion of the active layer 18 of the diode is not well utilized. Moreover, the conductive doping types for the electrode layer and the contact layer are different. It could cause a junction between them at the contact region, and affecting the operation of the diode.
The invention provides a structure of light emitting diode, which has a contact layer having structure of doped strained-layer supperlatices (SLS), so that the contact layer can easily have a high carrier concentration, resulting in high conductivity.
The invention provides a structure of light emitting diode, which uses strained-layer supperlatices structure to serve as a contact layer associating with the transparent electrode layer, so as to improve the light emitting efficiency and reduce the operation voltage.
The invention provides a structure of light emitting diode, which has a contact layer having structure of doped strained layer supperlatices (SLS). The dopant type in the contact layer is therefore not necessary to be restricted. The transparent electrode and the contact layer can even be the same conductive type, so that the junction between the transparent electrode and the contact layer is avoided.
The invention provides a structure of light emitting diode, which has a contact layer having structure of doped strained-layer supperlatices (SLS). The transparent electrode has better contact quality. The area of the transparent electrode can be about equal to the area of the active layer. This can allow the current to flow through the larger area of the active layer, so that the effective light emitting area is increased, and the light emitting efficiency is accordingly increased.
The invention provides a structure of light emitting diode, which is formed on a substrate, wherein an SLS structures is formed over a second confinement layer.
In the foregoing, the transparent electrode layer and the SLS layer can have different conductive types. The transparent electrode layer can have the conductive type of P-type or N-type.
The invention provides a structure of light emitting diode, having an SLS structure formed on the substrate.
In the foregoing, the transparent electrode layer and the SLS layer can be the same conductive type with all P-type or all N-type. The transparent electrode layer and the SLS layer can also be different conductive type.
It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed.